Short answers, pulled from the story.
German officials signed the Instrument of Surrender late in the evening on the 8th of May 1945. This document was witnessed by representatives from the United States and Free France.
The 9th of May became Victory Day because Moscow is five hours ahead of Central European Time when the signing occurred in Berlin-Karlshorst. The Soviet government announced the victory early on the 9th of May after the signing ceremony concluded.
Armenia recognized the 9th of May as Victory and Peace Day after independence in 1991 while Azerbaijan restored the holiday to its calendar in 1994. Belarus has held seven parades on Independence Square since 1995 and Kazakhstan celebrates the date alongside Defender of the Fatherland Day on the 7th of May.
Up to 12 million Russians participate in these marches nationwide. The Immortal Regiment was introduced in 2012 as a civil event where participants carry portraits of relatives who served.
Poland observed the 9th of May as National Victory and Freedom Day until 2015. The country shifted its official commemoration to the 8th of May that year.